Plasma enshrouded electric discharge device

ABSTRACT

The arc tube of a high intensity discharge lamp is disposed within a second arc tube which has a filling that is more easily startable than that of the first arc tube. A discharge struck between the electrodes of the second arc tube aids in starting a discharge between the electrodes of the first arc tube.

United States Patent [1 1 Keeffe et al.

PLASMA ENSHROUDED ELECTRIC DISCHARGE DEVICE Inventors: William M. Keeffe, Rockport; W.

Calvin Gungle, Danvers; Albert W.

Olson, Rockport, all of Mass.

Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

Filed: Aug. 30, 1973 Appl. No.: 393,022

US. Cl. 313/1, 3l3/l5;229 Int. Cl. H01j 61/22 Field of Search 313/1, 15, 229

References Cited UNITED STATES PATENTS 2/l940 Boucher 313/1 Aug. 6, 1974 Primary Examiner-H. K. Saalbach Assistant Examiner-Darwin R. l-lostetter Attorney, Agent, or FirmJames Theodosopoulos [5 7] ABSTRACT The arc tube of a high intensity discharge lamp is disposed within a second arc tube which has a filling that is more easily startable than that of the first arc tube. A discharge struck between the electrodes of the second arc tube aids in starting a discharge between the electrodes of the first arc tube.

9 Claims, 1 Drawing Figure PLASMA ENSHROUDED ELECTRIC DISCHARGE DEVICE BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to high intensity discharge lamps and particularly to high pressure sodium vapor lamps. Such lamps generally requires a high starting voltage to initiate a discharge between the electrodes thereof.

2. Description of the Prior Art High pressure sodium (HPS) vapor lamps have become commercially useful in recent years because their efficiency (lumens/watt) is generally higher than that of other high intensity discharge lamps such as high pressure mercury vapor or metal halide. One of the disadvantages of HPS lamps is the high starting voltage, generally between 2,000 to 3,000 volts, needed to initiate a discharge between the electrodes. Such a high starting voltage necessitates the use of a special expensive ballast.

Recently, HPS lamps have become available that do not require such high starting voltages nor the special ballast associated therewith. Examples of these lamps are shown in U.S. Pat. Nos. 3,721,845, 3,721,846 and 3,746,914. Such lamps require one or more of the following to lower starting voltage: a wire wound heater to heat the arc tube; a starting aid or conducting plane that comprises a wire encircling or adjacent the arc tube; selection of the starting gas or gases for low voltage startability.

There are disadvantages associated with each of these methods of lowering starting voltage. In the case of the wire wound heater, it is necessary to electrically disconnect the heater after the arc tube hasignited, in order to prevent low lamp efficiency. It is also necessary or desirable to disconnect the wire starting aid, after ignition, to prevent sodium migration through the arc tube wall. Such disconnect requirements necessitate the use of thermal switches, which can reduce lamp reliability and life.

In the case of a starting gas selected for low voltage startability, lamp efficiency is sacrificed. As pointed out in U.S. Pat. No. 3,248,590, the heavier inert gases provide higher efficiency than lighter inert gases such as neon which is sometimes used to aid startability.

The instant invention concerns a plasma enshrouded 50 electric discharge device which overcomes the disadvantages mentioned above.

SUMMARY OF THE INVENTION An electric discharge device, in accordance with this invention, comprises a first arc tube having electrodes at its ends disposed within a second arc tube also having electrodes at its ends, which are electrically connected in parallel with the electrodes of the first arc tube. The second arc tube has a filling that ignites at a lower voltage than that of the first arc tube. Upon igni- 1 first arc tube, since the voltage applied to the first arc tube is the voltage drop of the discharge in the second arc tube, as a consequence of the two tubes being electrically connected in parallel, aided by the fact that said plasma acts as a conducting plane. Also, since the sustaining voltage of the arc in the second arc tube is greater than the operating voltage at ignition or shortly thereafter of the arc in the first arc tube, the enshrouding discharge is extinguished.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE in the drawing is an elevational view, partly in section, of an electric discharge lamp in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawing an electric discharge lamp in accordance with this invention comprises an arc tube 1 having electrodes 2 and 3 disposed at its ends. Arc tube 1 is disposed within a second arc tube 4 having electrodes 5 and 6 disposed at its ends. Arc tube 4 may have a starter electrode 7 adjacent main electrode 5 to aid in starting an electric discharge in arc tube 4.

Arc tube 3 is disposed within an outer glass envelope 8 and is supported therein by means of metal frame 9 which is fastened to lead-in wire 10 which is embedded in stem press 11 at one end of envelope 8. Clamp 12 is mounted on frame 9 and holds press seal 13 of arc tube 4. The upper end of arc tube 4 is supported by a similar metal frame having a clamp 15 holding press seal 16 of arc tube 4. Metal frame 14 is supported within the upper smaller diameter end of envelope 8 by metal leaf springs 17 which bear against the inner wall thereof.

Electrical connection is provided from lead-in wire 10 through frame 9 to electrodes 3 and 6 by wires 18 and 19 which are connected to frame 9. Electrical connection is provided from lead-in wire 20, also embedded in stem press 11, through frame 14 by wire 21 to electrodes 2 and 5 which are connected to frame 14 by wires 22 and 23. Starter electrode 7 is connected to lead-in wire 20 through resistor 24 and aids in starting an electrical discharge in arc tube 4, as is known in the prior art, shown, for example, in U.S. Pat. No. 3,445,721. Lead-in wires 10 and 20 protrude through press 11 and are connected to theusual screw type base 24.

As previously mentioned, arc tube 4 contains a fill that ignites at a lower voltage than the fill in arc tube 1. However ignition voltage of the fill in arc tube 1 decreases as the temperature thereof is increased. A temperature is reached where the voltage between electrodes land 3 is sufficient to ignite arc tube 1, aided by the fact that the discharge plasma in arc tube 4 acts as a conducting plane or starting aid. And, since the sustaining voltage of the arc discharge in arc tube 4 is greater than the operating voltage of the arc discharge in arc tube 1, the arc discharge in arc tube 4 is extinguished.

In one example of a lamp in accordance with the invention, arc tube 1 was a standard are tube for a 400 watt high pressure sodium lamp. The are tube was made of alumina, had-the usual niobium end cap seals with activated tungsten electrodes at each end and contained a fill of mercury and sodium and a starting gas of xenon at a pressure of 13 Torr. The normal ignition voltage for such an arc tube is about 2,500 volts.

Are tube 4 was made from 22 by 25 mm quartz tubing, had a tube length of 150 mm and an arc length of 128 mm. Electrodes and 6 were barium, calcium, thorium, oxide banner cathodes and are tube 4 had a fill of 200 mg mercury and a starting gas of argon at a pressure of 18 Torr.

The lamp was connected to a ballast circuit that provided an open circuit voltage of 150 volts RMS having a sinusoidal wave form. This voltage ignited a discharge in arc tube 4 between electrodes 5 and 6. The voltage drop between electrodes 5 and 6, immediately after ignition, was very low, say, about 30 volts. As the discharge heated arc tube 4 and vaporized more mercury therein, the voltage drop between electrodes 5 and 6 increased. At the same time, the discharge plasma in arc tube 4 partially or completely enshrouded arc tube 1 and gradually raised the temperature thereof. After about 45 seconds, are tube 1 had been heated sufficiently so that the voltage drop between electrodes 5 and 6 was sufficient to ignite a discharge between electrodes 2 and 3. And, since the operating voltage of the arc discharge between electrodes 2 and 3 was less than the sustaining voltage of the arc discharge between electrodes 5 and 6, the arc discharge between electrodes 5 and 6 was extinguished.

We claim:

1. An arc discharge device comprising a first arc tube disposed within a second arc tube, the second arc tube containing a fill that is startable at a lower voltage than that of the first arc tube, the first arc tube containing a fill the starting voltage of which decreases with increasing arc tube temperature, the operating voltage of tubes has electrodes at each end thereof, each electrode in the first arc tube being electrically connected to the proximate electrode in the second arc tube.

3. The device of claim 1 wherein the first arc tube comprises an alumina tube having a fill including mercury, sodium and a starting gas.

4. The device of claim 3 wherein the second arc tube has a fill including mercury and a starting gas.

5. The device of claim 1 wherein the discharge in the second arc tube, after starting thereof, enshrouds the first arc tube.

6. The device of claim 5 wherein said enshrouding discharge raises' the temperature of the first arc tube and reduces the starting voltage thereof.

7. The method of starting an arc discharge in an arc tube by enshrouding said arc tube in the discharge plasma of a second arc tube and heating the first arc tube to a temperature at which the voltage across the second arc tube is sufficient to start an arc discharge in the first arc tube.

8. The method of starting an arc discharge in an arc tube of a high pressure sodium vapor lamp by disposing said are tube in a second arc tube having a lower starting voltage, starting an arc discharge in the second arc tube which heats up the first arc tube to startability, and extinguishing the. arc discharge in the second arc tube after starting of the first arc tube.

9. A high pressure sodium vapor lamp comprising an arc tube, having a fill including sodium, mercury and a starting gas, disposed within a second arc tube having a fill including mercury and a starting gas, the starting voltage of the second arc tube being lower than that of the first arc tube. 

1. An arc discharge device comprising a first arc tube disposed within a second arc tube, the second arc tube containing a fill that is startable at a lower voltage than that of the first arc tube, the first arc tube containing a fill the starting voltage of which decreases with increasing arc tube temperature, the operating voltage of the first arc tube, after starting thereof, being less than the sustaining voltage of the second arc tube.
 2. The device of claim 1 wherein each of said arc tubes has electrodes at each end thereof, each electrode in the first arc tube being electrically connected to the proximate electrode in the second arc tube.
 3. The device of claim 1 wherein the first arc tube comprises an alumina tube having a fill including mercury, sodium and a starting gas.
 4. The device of claim 3 wherein the second arc tube has a fill including mercury and a starting gas.
 5. The device of claim 1 wherein the discharge in the second arc tube, after starting thereof, enshrouds the first arc tube.
 6. The device of claim 5 wherein said enshrouding discharge raises the temperature of the first arc tube and reduces the starting voltage thereof.
 7. The method of starting an arc discharge in an arc tube by enshrouding said arc tube in the discharge plasma of a second arc tube and heating the first arc tube to a temperature at which the voltage across the second arc tube is sufficient to start an arc discharge in the first arc tube.
 8. The method of starting an arc discharge in an arc tube of a high pressure sodium vapor lamp by disposing said arc tube in a second arc tube having a lower starting voltage, starting an arc discharge in the second arc tube which heats up the first arc tube to startability, and extinguishing the arc discharge in the second arc tube after starting of the first arc tube.
 9. A high pressure sodium vapor lamp comprising an arc tube, having a fill including sodium, mercury and a starting gas, disposed within a second arc tube having a fill including mercury and a starting gas, the starting voltage of the second arc tube being lower than that of the first arc tube. 